(a) Field of the Invention
The present invention relates generally to attitude determination systems and, more particularly, to a method and apparatus for autonomous attitude acquisition for a stellar inertial attitude determination system.
(b) Description of Related Art
The term attitude is used to describe the orientation of an object with respect to a reference orientation. Attitude is of particular interest in satellite operations. For example, if a satellite is to be used in a communications application, it is necessary that the satellite be oriented in the proper direction to receive and/or transmit relevant information for the communication link. The attitude of a satellite is determined by computations based on the output of attitude sensors located on the satellite. Attitude sensors monitor the position of reference objects, which may include the sun, the earth, stars and/or star constellations, or radio frequency beacons. For example, a spacecraft may have two or more sensors, one monitoring the sun and one monitoring the earth. However, sun sensors are ineffective when the sun cannot be observed (i.e., when the earth is between the satellite and the sun). Better availability of multiple stars in the sky makes star trackers the preferred attitude measurement devices in many spacecraft applications.
A spacecraft may use two or more star trackers to monitor the position of constellations or stars with respect to the spacecraft. Attitude is determined by comparing information from the star trackers to information in an on-board star catalog (OSC). An OSC includes information such as direction to the stars (right ascension or declination) in the ECI frame and the magnitude of brightness of certain stars. Stars observed by the star trackers that are located in the OSC are commonly referred to as OSC stars. A comparison between where stars are sensed, with respect to the satellite, by the star tracker and the stars in the OSC enables the satellite to determine its attitude.
Comparisons between sensed stars and OSC stars may be made in two different ways: pattern matching and direct matching. When a satellite or spacecraft has no information about its attitude, the spacecraft is "lost in space." When a spacecraft is lost in space, it must acquire its attitude in order to begin its mission. Attitude acquisition is typically performed through pattern matching. To perform pattern matching, the lost spacecraft observes constellations and compares the constellations observed using its star trackers to the stars in the OSC. Various pattern matching methods are disclosed in several U.S. patents. For example, U.S. Pat. Nos. 4,621,329 and 4,944,587 to Jacob and Harigae, respectively, disclose pattern matching methods based on processing a single snap-shot of the sky taken by star trackers. Some pattern matching approaches use a priori information of spacecraft position to reduce the size of the OSC needed. Exemplary systems are disclosed in U.S. Pat. Nos. 4,658,361 and 5,177,686 to Kosaka et al. and Boinghoff et al., respectively.
Once attitude acquisition is performed, spacecraft attitude is basically known within some tolerance. Once this a priori information is known, a direct match procedure may be used to fine-tune and maintain the attitude of the spacecraft. Direct matching uses star trackers to observe stars and examines the observed stars to find a direct match between the observed stars and the OSC stars. When a direct match is found, a reference vector to the star in the ECI frame is calculated and spacecraft attitude is precisely determined.
Star trackers vary in both sensitivity and field-of-view (FOV). Field-of-view is a measure of how broad an area of sky a tracker can monitor. The sensitivity of a star tracker determines how bright a star must be before it can be detected by the tracker. The FOV and sensitivity of a star tracker dictate how many stars that tracker can observe at a given instant in time. To acquire attitude using star trackers and a pattern matching method, at least three OSC stars must be observed at one time. The use of narrow FOV sensors or sensors with reduced sensitivity lowers the probability that sufficient number of OSC stars will be observable by the star trackers. If sufficient OSC stars are not available in the tracker FOV, attitude cannot be acquired using pattern matching.
Disclosed prior systems, which use only a single snap-shot of the sky, require a sufficient number of stars in the star tracker FOV in order to acquire spacecraft attitude. Therefore, in many instances, spacecraft attitude may not be acquired due to insufficient OSC stars in the star tracker FOV. Accordingly, there is a need for a star-based attitude acquisition method that provides attitude information when insufficient OSC stars are present in the star tracker field-of-view in a single measurement.